PCSK9 Regulates Cardiac Mitochondrial Cholesterol by Promoting TSPO Degradation

Cholesterol is critical for mitochondrial membrane structure and function. Given the emergence of mitochondria as a key factor in the pathogenesis of heart failure, mitochondrial cholesterol homeostasis may be crucial for maintaining mitochondrial properties and thus cardiac function. We previously showed that CM-Pcsk9^-/- mice (mice with cardiomyocyte-specific deletion of the gene encoding PCSK9 [proprotein convertase subtilisin-kexin type 9]) have impaired cardiomyocyte mitochondrial bioenergetics and heart function, paralleled by cardiomyocyte mitochondrial cholesterol accumulation and an increased number of mitochondria-endoplasmic reticulum contacts. However, the mechanisms linking PCSK9 to mitochondrial cholesterol homeostasis remain unclear. We hypothesized that PCSK9 acts on proteins involved in mitochondrial cholesterol trafficking in the heart to maintain cardiac mitochondrial function.

By performing RNA sequencing and immunoblot on hearts from CM-Pcsk9^-/- and CM-Pcsk9^+/+ (without cardiomyocyte-specific deletion of Pcsk9) mice, we showed that TSPO (translocator protein) was increased by Pcsk9 deficiency. To investigate the relationship between TSPO levels and heart function in humans, we compared the transcriptome of human left ventricles with high versus low TSPO levels. We used H9c2 (a rat cardiomyoblast cell line) cardiomyocytes to explore the mechanism linking PCSK9/TSPO to mitochondrial cholesterol content and function. The impact of reduced TSPO levels on cardiac function and mitochondrial oxidation in CM-Pcsk9^-/- mice was tested using adeno-associated virus serotype 9 short hairpin TSPO.

Both gene and protein levels of TSPO, a mitochondrial protein involved in cholesterol transport, were increased in CM-Pcsk9^-/- mouse hearts. Transcriptome analysis showed that high TSPO expression in human left ventricles was associated with impaired mitochondrial and cardiac function. We showed that PCSK9 induced TSPO degradation through a proteasomal mechanism that occurs in cardiomyocytes but not hepatocytes and contributes to maintaining normal mitochondrial cholesterol composition and function. At the molecular level, endoplasmic reticulum-resident PCSK9 interacted with GRP78 (glucose regulatory protein 78) , reducing GRP78-TSPO interactions and leading to TSPO misfolding and degradation by the ubiquitin-proteasome pathway. Importantly, gene therapy-induced downregulation of TSPO in CM-Pcsk9^-/- mice prevented mitochondrial cholesterol accumulation and improved cardiac function.

These findings indicate that PCSK9 regulates mitochondrial cholesterol levels by modulating the TSPO degradation in the heart. Modulation of mitochondrial cholesterol by targeting TSPO may be a promising therapeutic approach for heart failure.